Thermal stress reduction in aircraft motor controllers

ABSTRACT

A thermal stress reduction method includes ramping an electric power generator to start an aircraft engine, for a time period associated with the aircraft engine start sequence toggling a three-level inverter switch array to a three-level pulse width modulation mode, determining if a first time interval in the three-level pulse width modulation mode exceeded a predetermined three-level pulse width modulation mode interval, in response to the first time interval exceeding the three-level pulse width modulation mode interval, toggling the three-level inverter switch array to a two-level pulse width modulation mode, determining if a second time interval in the two-level pulse width modulation mode exceeded a predetermined two-level pulse width modulation mode interval and in response to the second time interval exceeding the two-level pulse width modulation mode interval, toggling the three-level inverter switch array to the three-level pulse width modulation mode.

BACKGROUND OF THE INVENTION

The present invention relates to aircraft electric start motors, andmore specifically, to motor controllers for managing thermal stressassociated with electrical start of aircraft engines.

The engines in an aircraft are typically started by non-electricalmethods, for example, a start turbine driven by compressed air. Anincreased number of aircrafts in recent years have begun to use electricgenerators to start the engine by operating the generator in motoringmode, powered by a motor controller. The motor controllers on aircraftare typically designed for other continuous duty applications, such as,cabin air compressor, air recirculation fans, and fuel pumps. When thereis a need for an engine start, typically one or two of the availablemotor controllers are called upon from their normal duty to start theengine, and then returned to their normal duty. Because of the very highthermal stress to the motor controller semiconductor power switches(e.g., an insulated gate bipolar transistor (IGBT) switch) during anengine start, however brief this engine start duty may be, the IGBTswitch in conventional motor controllers are sized for this brief enginestart duty, or an additional motor controller is used in parallel. As aresult, the motor controller becomes heavier, or if parallel motorcontrollers are used, the system weight increases due to additionalpower feeders and contactors.

BRIEF DESCRIPTION OF THE INVENTION

Exemplary embodiments include, during an aircraft engine start sequence,a thermal stress reduction method for an aircraft motor controllerhaving a three-level inverter switch array, and configured to control anelectric power starter/generator coupled to an aircraft engine. Themethod includes ramping the electric power starter/generator to startthe aircraft engine, for a time period associated with the aircraftengine start sequence toggling the three-level inverter switch array toa three-level pulse width modulation mode, determining if a first timeinterval in the three-level pulse width modulation mode exceeded apredetermined three-level pulse width modulation mode interval. If thefirst time interval exceeds the three-level pulse width modulation modeinterval, the method further includes toggling the three-level inverterswitch array to a two-level pulse width modulation mode, determining ifa second time interval in the two-level pulse width modulation modeexceeded a predetermined two-level pulse width modulation mode intervaland in response to the second time interval exceeding the two-levelpulse width modulation mode interval, toggling the three-level inverterswitch array to the three-level pulse width modulation mode.

Additional exemplary embodiments include an engine start system with anelectric power starter/generator, an aircraft engine coupled to theelectric power starter/generator, an aircraft motor controller coupledto the electric power generator. The system further includes a thermalstress reduction process residing on the motor controller and configuredto ramp the electric power starter/generator to start the aircraftengine, for a time period associated with the aircraft engine startsequence toggle the three-level inverter switch array to a three-levelpulse width modulation mode, determine if a first time interval in thethree-level pulse width modulation mode exceeded a predeterminedthree-level pulse width modulation mode interval. The process is furtherconfigured to, in response to the first time interval exceeding thethree-level pulse width modulation mode interval, toggle the three-levelinverter switch array to a two-level pulse width modulation mode,determine if a second time interval in the two-level pulse widthmodulation mode exceeded a predetermined two-level pulse widthmodulation mode interval, and in response to the second time intervalexceeding the two-level pulse width modulation mode interval, toggle thethree-level inverter switch array to the three-level pulse widthmodulation mode.

Additional exemplary embodiments include, during an aircraft enginestart sequence, a thermal stress reduction method for an aircraft motorcontroller having a three-level inverter switch array, and configured tocontrol an electric power starter/generator coupled to an aircraftengine. The method includes ramping the electric power generator tostart the aircraft engine, and for a time period associated with theaircraft engine start sequence, applying a current profile to thecurrent from the electric power generator to the aircraft engine. Thecurrent profile controls the current to reduce the thermal dissipationin the three-level inverter switch.

Additional exemplary embodiments include an engine start system with anelectric power starter/generator having a three-level inverter, anaircraft engine coupled to the electric power generator, an aircraftmotor controller coupled to the electric power generator, a thermalstress reduction process residing on the motor controller and configuredto ramp the electric power generator to start the aircraft engine andfor a time period associated with the aircraft engine start sequence,apply a current profile to the current from the electric power generatorto the aircraft engine, the current profile controlling the current toreduce the thermal dissipation in the three-level inverter switch.

Additional exemplary embodiments include, during an aircraft enginestart sequence, a thermal stress reduction method for an aircraft motorcontroller having a three-level inverter switch array, and configured tocontrol an electric power generator coupled to an aircraft engine, themethod including ramping the electric power generator to start theaircraft engine and for a time period associated with the aircraftengine start sequence, ramping an output frequency of the motorcontroller from an initial frequency, and controlling the outputfrequency to reduce the thermal dissipation in the three-level inverterswitch.

Further exemplary embodiments include an engine start system, includingan electric power starter/generator having a three-level inverter, anaircraft engine coupled to the electric power generator, an aircraftmotor controller coupled to the electric power generator, a thermalstress reduction process residing on the motor controller and configuredto ramp the electric power generator to start the aircraft engine andfor a time period associated with the aircraft engine start sequence,ramping an output frequency of the motor controller from an initialfrequency, controlling the output frequency to reduce the thermaldissipation in the three-level inverter switch.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 illustrates a schematic diagram of an engine start circuit inwhich the exemplary methods and systems described herein areimplemented;

FIG. 2 schematically illustrates an engine start circuit in whichexemplary embodiments may be implemented;

FIG. 3 illustrates a three-level inverter IGBT switch array;

FIG. 4 illustrates an interconnection of four semiconductor switches ofone leg in a three-level phase motor controller;

FIG. 5 illustrates the switch array of FIG. 3 in a three-level pulsewidth modulation mode.

FIG. 6 illustrates the switch array of FIG. 3 in a two-level pulse widthmodulation mode;

FIG. 7 illustrates an example of a plot of switch gate control signalsfor one phase that illustrate switching between the two-level pulsewidth modulation mode and the three-level pulse width modulation mode inaccordance with exemplary embodiments;

FIG. 8 illustrates a flowchart for a method of toggling between atwo-level pulse width modulation mode and a three-level pulse widthmodulation mode in accordance with exemplary embodiments;

FIG. 9 illustrates a three-level inverter IGBT switch array in whichexemplary embodiments for applying a current control method can beimplemented;

FIG. 10 illustrates a flowchart for a method of applying a currentprofile to a motor controller in accordance with exemplary embodiments;and

FIG. 11 illustrates a flowchart for a method of applying a frequencyramp to a motor controller in accordance with exemplary embodiments.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a schematic diagram of an engine start circuit 100 inwhich the exemplary methods and systems described herein areimplemented. Driven by the motor controller, the starter/generator spinsup with or without the engine engaged. As described herein, thegenerators are implemented for various tasks on the aircraft, and canalso be implemented to start the engines. The curve 101 represents theapplied power and the curve 102 represents the resultant speed. During afirst period 110, the generator applies a constant torque for a speedramp. During a second time period 120, the generator provides constantpower with a constant speed. During a third time period 130, thegenerator ramps down the applied power and the speed remains constant.During the third time period, frequency control is typicallyimplemented, synchronizing the starter/generator frequency to busfrequency and bypassing control from the motor controller. During afourth time period, 140, the speed stays constant and the engines areengaged and begin acceleration. During a fifth and sixth time periodsthe engines ignite and become fully functional.

FIG. 2 schematically illustrates an engine start circuit 200 in whichexemplary embodiments may be implemented. A motor controller 210 is fedfrom a variable frequency/constant frequency (VF/CF) bus 220 through acontactor/breaker 230. The output of motor controller 210 is connectedto starter/generator 240 through a contactor/breaker 250, which is inparallel with a contactor/breaker 260. The motor controller 210 iscoupled to the generator 240 via a generator control unit (GCU) 270. Thestarter/generator 240 is coupled to the aircraft engine 280 via suitabletransmission and coupling 290. After the generator 240 spins up to nearsynchronous speed and is synchronized with the bus, thecontactor/breaker 250 opens and contactor/breaker 260 closes.

As described herein, the electric power generators are also driven bythe aircraft engines to generate electricity for power distributionwithin the aircraft. In a more electric aircraft, the engines are oftenstarted by using these generators in motoring mode, thus eliminatingneed for air-driven turbine starters. The spin up of the generator laststypically from a few seconds to tens of seconds, depending on the enginesize and if the engine is engaged when spinning the generator.Consequently there is no dedicated engine start motor controller.Instead, a motor controller installed for driving other normal dutyaircraft loads may be called upon to perform this short engine startduty.

FIG. 3 illustrates a three-level inverter IGBT switch array 300 in whichexemplary embodiments for toggling between a two-level pulse widthmodulation (PWM mode and a three-level PWM mode may be implemented. Itcan be appreciated that other power switches can be implemented in otherexemplary embodiments. The array 300 includes three legs 310, 320, 330,each leg 310, 320, 330 including one or more IGBT switches. As describedherein, the IGBT switch is implemented with the motor controllers (e.g.,the motor controller 210 in FIG. 2) of aircraft. In one embodiment, athree-level inverter motor drive PWM control method is implemented tospread the power semiconductor (e.g. IGBT) thermal stress more evenlyduring an electrical start of the aircraft engines. When a motorcontroller is used to ramp up the electric starter/generator machine forthe purpose of engine start in a more electric aircraft, one of themotor controller power semiconductor switch (e.g., IGBT) junction couldbe subject to a very high thermal stress for a short duration, typicallyunder a second), while the other semiconductor switches are lessstressed or even not stressed at all during this same period. The motorcontroller IGBT switches would have to be sized based on this briefundistributed thermal stress resulting in undesirable weight increase.The engine start duty stresses the motor controller differently ascompared to a continuous motoring load. Significant among thedifferences is the very high thermal and power stresses to the IGBTswitch within the first quarter electrical cycle (i.e., the first timeperiod 110 of FIG. 1). The duration of this peak stress is typicallyshorter than one second. FIG. 3 illustrates, for example, that duringpeak current conditions in the motor controller, two switches 311 in thefirst leg 310 experience the highest thermal stress. During half peakcurrent conditions, two of the switches 321, 331 in the second and thirdlegs 320, 330, respectively, experience the highest thermal stress. Theexemplary methods described herein spread the thermal stresses moreevenly during engine start-up. Within the first quarter (e.g., the firsttime period 110 of FIG. 1) of this very low frequency electrical cycle(starter/generator being spun up from zero speed), the IGBT switchjunction temperature rise can double or even triple in comparison tonormal steady state (higher frequency) operation. Increasing the IGBTswitch size and weight, and hence the overall motor controller packagesize and weight, for the sake of this short duty-cycle (one minute)engine start can be addressed with the methods described herein withoutchanging the motor controller package size and weight.

FIG. 4 illustrates an interconnection of four semiconductor switches S1,S2, S3, S4 of one leg 400 in a three-level phase motor controller. Forexample, the leg 400 can be any of the legs 310, 320, 330 of FIG. 3. Forillustrative purposes switches S1, S4 are referred to as “outer”switches, and switches S2, S3 are referred to as “inner” switches.Typically, the starter/generator (e.g., the generator 240 in FIG. 2) isof the wound-field synchronous type. The engine (e.g., the engine 280 inFIG. 2) may or may not be connected to the generator when spinning upthe generator. The motor controller output is a three phase ac voltagewith a frequency ramp up from 0 Hz (or near 0 Hz) to 400 Hz. Because ofthe slow frequency ramp, one of the inverter switches could experience aswitching loss of nearly 3.14 times of that during normal high frequencysteady-state operation, in a two-level inverter or three-level inverter.The IGBT switch thermal time constant is typically less than 100 ms. Itis normally the first output cycle sinusoidal peak that thermallystresses the IGBT switch the most during this engine start-up. Duringthis first half of the output frequency cycle, one of the outer switchesis continually switching and the adjacent inner switch is in a steady ONstate. This results in the outer switch being highly stressed thermally,due to the high incurred switching losses, while the inner switch islightly thermally stressed, since the inner switch incurs no switchingloss. By implementing the PWM methods described herein, the outer andinner IGBT switches in a three-level inverter are switched alternatelyso the thermal stress to a particular IGBT is reduced significantlyresulting in a reasonable rating for the switches during electricalengine start. As described herein, the PWM method toggles between athree-level PWM (when the outer switches perform the PWM switching) modeand a two-level PWM (when the inner switches perform the PWM switching)mode, thus more evenly spreading the number of switching instantsbetween the outer and inner switches. The method reduces the outerswitches thermal stress by shifting part of the thermal loss to theinner switches

As an example, a period near when the output current from a phase A isat its positive peaks, the highest current passes through switches S1,S2. In a convention three-level PWM mode, the outer switches would begated for high frequency switching, and the inner switches would beoperated as continually on or off. Consequently, the outer switcheswould produce considerably higher losses than the inner switches. FIG. 5illustrates the switch array 300 of FIG. 3 in a three-level PWM mode. Apulsing scheme chart 500 for a three-level PWM mode is also illustrated.FIG. 5 illustrates peak current switching ON in switch S1, peak currentON in switch S2, in leg 310, and half peak current ON in switch S3 andhalf peak current switching ON in switch S4 in the legs 320, 330. In theexample shown in FIG. 5, S1 in Phase A would be stressed the most. Inone case, the outer switch S1 junction temperature could reach 168 Cduring the first cycle of engine start and the inner switch junctiontemperature would only reach 98 C, a difference of 70 C.

FIG. 6 illustrates the switch array 300 of FIG. 3 in a two-level PWMmode. A pulsing scheme chart 600 for a two-level PWM mode is alsoillustrated. FIG. 6 illustrates peak current ON in switch S1, peakcurrent switching ON in switch S2, in leg 310, and half peak currentswitching ON in switch S3 and half peak current ON in switch S4 in thelegs 320, 330. If the outer switches are forced to stay in the ON state,and the inner switches are utilized for PWM switching, as illustrated inFIG. 6, the losses on outer switches would be significantly reduced.However, the inner switches are dissipating higher losses in this modeof operation.

In one embodiment, by toggling between the two PWM modes illustrated inFIGS. 5 and 6, in a time interval shorter than the semiconductor switch(e.g. IGBT) thermal time constant, the excessive thermal stress normallyexperienced by only the outer switches during engine starts would now bedistributed between the outer and inner switches therefore greatlyreducing the thermal stress of individual switches. FIG. 7 illustratesan example of a plot 700 of switch gate control signals for one phasethat illustrate switching between the two-level PWM mode and thethree-level PWM mode in accordance with exemplary embodiments. Theswitching losses for the three-level and the two-level PWM modes aredifferent. As such, a ratio is selected for the time intervals for thethree-level PWM mode and the two-level PWM mode to be inverselyproportional to the ratio of the three-level PWM mode loss in the switchversus the two-level PWM mode loss in the switch. As a result of thetoggling between the two-level and three-level PWM modes, a reduction ofS1 junction temperature to 145 C as compared to 168 C in conventionalPWM modes was achieved. The inner switch S2 junction temperatureincreases to 150 C, a 5 C difference, therefore a more even thermaldistribution.

In one embodiment, higher switching frequencies or carrier frequenciescan be used in the two-level PWM mode to reduce common mode voltageoutput if a common mode filter is used, thereby decreasing the weightand size of the output common-mode filters. In one embodiment, thetwo-level PWM mode alternating time period is less than thesemiconductor thermal time constant of the switches.

FIG. 8 illustrates a flowchart for a method 800 of toggling between atwo-level PWM mode and a three-level PWM mode in accordance withexemplary embodiments. At block 810, the motor controller (e.g., themotor controller 210 of FIG. 2) enters into a three-level PWM mode atwhich time a timer is set for the three-level PWM mode. At block 820,the motor controller determines whether the elapsed time in thethree-level PWM mode has surpassed a pre-determined three-level PWM modeinterval. During this time, an increased amount of thermal (power)dissipation occurs on switches S1, S4 as described herein. Thepredetermined three-level PWM mode interval is selected to switch fromthe three-level PWM mode before the switches S1, S4 overheat. If thetime interval in the three-level PWM mode has not surpassed thepre-determined three-level PWM mode interval at block 820, then themotor controller stays in the three-level PWM mode at block 810. If thetime interval in the three-level PWM mode has surpassed thepre-determined three-level PWM mode interval at block 820, then themotor controller enters into a two-level PWM mode at block 830, in whicha timer is set for the two-level PWM mode. At block 840, the motorcontroller determines whether the elapsed time in the two-level PWM modehas surpassed a pre-determined two-level PWM mode interval. During thistime, an increased amount of thermal (power) dissipation occurs onswitches S2, S3 as described herein. The predetermined two-level PWMmode interval is selected to switch from the three-level PWM mode beforethe switches S2, S3 overheat. If the time interval in the two-level PWMmode has not surpassed the pre-determined two-level PWM mode interval atblock 840, then the motor controller stays in the two-level PWM mode atblock 830. If the time interval in the two-level PWM mode has surpassedthe pre-determined two-level PWM mode interval at block 840, then themotor controller enters back into a three-level PWM mode at block 830,in which a timer is set for the three-level PWM mode. It can beappreciated that this method 800 repeats thereby toggling between thetwo-level PWM mode and the three-level PWM mode during the enginestart-up as described herein.

In another embodiment, a current profile method that includes a motorcontroller current profile that reduces IGBT thermal stress during anelectrical start of the main engine can be implemented. When the motorcontroller (e.g., the motor controller 210 of FIG. 2) is used to ramp upthe electric generator machine for the purpose of engine start in a moreelectric aircraft, the power semiconductor switch (e.g., IGBT) junctionis subject to a very high thermal stress (for a short durations under asecond). The motor controller IGBT switch would have to be sized basedon this brief thermal stress resulting in undesirable weight increase.By using the motor controller output current profile described herein,the IGBT switch thermal stress is reduced significantly so that IGBTswitch size increase would not be necessary for engine start duty, hencereducing the aircraft weight.

The motor controller designed for a certain continuous motor load can beused to spin the generator machine up to 400 Hz to start the engine.Typically, the generator is of the wound-field synchronous type. Theengine may or may not be connected to the generator when spinning thegenerator. The motor controller output is a three phase AC voltage witha frequency ramp up from zero or close to zero frequency to 400 Hz.Because of the slow frequency ramp, one of the IGBT switch couldexperience a switching loss of nearly 3.14 times of that during a highfrequency steady operation. In addition, the IGBT thermal time constantis less than 100 ms. Typically, the first output cycle sinusoidal peakthermally stresses the IGBT switch the most compared to subsequentpeaks. Therefore, if the current is reduced during the first quartercycle, the IGBT thermal stress would be reduced significantly. Becausethe current is only reduced for a very short duration the overallgenerator acceleration time is not noticeably affected.

FIG. 9 illustrates a three-level inverter IGBT switch array 900 in whichexemplary embodiments for applying a current control method can beimplemented. The array 900 includes three legs 910, 920, 930, each leg910, 920, 930 including one or more IGBT switches. As described herein,the IGBT switch is implemented with the motor controllers (e.g., themotor controller 210 in FIG. 2) of aircraft. In one embodiment, acurrent control method is implemented to spread the power semiconductor(e.g. IGBT) thermal stress more evenly during an electrical start of theaircraft engines. The engine start duty stresses the motor controllerdifferently as compared to a continuous motoring load. Significant amongthe differences is the very high thermal and power stresses to the IGBTswitch within the first quarter electrical cycle (i.e., the first timeperiod 110 of FIG. 1). The duration of this peak stress is typicallyshorter than one second. FIG. 9 illustrates, for example, that duringpeak current conditions in the motor controller, one switch 911 in thefirst leg 910 experiences the highest thermal stress. During half peakcurrent conditions, a respective switch 921, 931 in the second and thirdlegs 920, 930, respectively, experience the highest thermal stress. Theexemplary methods described herein spread the thermal stresses moreevenly during engine start-up. Within the first quarter cycle of theoutput (e.g., the first time period 110 of FIG. 1) of this electrical,the IGBT switch junction temperature rise, can double or even triple incomparison to normal steady state (higher frequency) operation.Increasing the IGBT switch size and weight, and hence the overall motorcontroller package size and weight, for the sake of this shortduty-cycle (one minute) engine start can be addressed with the methodsdescribed herein without changing the motor controller package size andweight.

As described herein, the IGBT switch peak junction temperature issignificantly reduced by controlling the motor current magnitudeprofile. In one embodiment, if the current magnitude starts at 0.5 perunit (pu) and increases to 1.0 pu in 0.6 seconds, the peak IGBT switchjunction temperature could be reduced by nearly 30° C. As such, byadjusting the rate of current increase per unit time, temperaturereduction in the IGBT is attained.

In addition, the IGBT switch junction temperature reaches the peak ataround the first current peak and decays nearly exponentially. Thereforeif the current profile is increased following a near exponential curve,a low flat peak temperature of the IGBT switch is achieved whilemaximizing the available torque for accelerating the generator. Another12° C. of IGBT switch temperature reduction is achieved by implementingan exponential current profile.

If in addition, a frequency ramp is from a non-zero frequency, furthertemperature reduction of the IGBT switch can be achieved. Alternatively,motor controller output current could ramp from a higher initial valueto boost acceleration torque.

In another embodiment, when necessary for the purpose of rotor polealignment, a dwell time at the initial non-zero frequency can also beintroduced. The non-zero frequency allows time for the rotor to catch upwith the stator field. Synchronous and induction torques enable rotorpole alignment. By combining with the current profile, which controlsthe current increase over a period of time, in addition to adding thedwell time, the rotor can catch up to synchronization.

By using this controlled exponential current ramp profile, and the dwelltime as described herein, a motor controller designed for other motoringload applications can be used for engine start without increasing theIGBT switch size.

FIG. 10 illustrates a flowchart for a method 1000 of applying a currentprofile to a motor controller in accordance with exemplary embodiments.At block 1010, during engine startup implementing a motor controller, anappropriate current profile as described herein is programmed into themotor controller.

In another embodiment, a frequency ramp method reduces IGBT thermalstress during an electrical start of the main engine can be implemented.When the motor controller (e.g., the motor controller 210 of FIG. 2) isused to ramp up the electric generator machine for the purpose of enginestart in a more electric aircraft, the power semiconductor switch (e.g.,IGBT) junction is subject to a very high thermal stress (for a shortdurations under a second). The motor controller IGBT switch would haveto be sized based on this brief thermal stress resulting in undesirableweight increase. By using the frequency ramp profile described herein,the IGBT switch thermal stress is reduced significantly so that IGBTswitch size increase would not be necessary for engine start duty, hencereducing the aircraft weight.

As described herein, the motor controller designed for a certaincontinuous motor load can be used to spin the generator up to 400 Hz tostart the engine. The engine may or may not be connected to thegenerator when spinning the generator. The motor controller output is athree phase AC voltage with a frequency ramps up from zero or close tozero frequency to 400 Hz. Because of the slow frequency ramp, one of theIGBT could experience a switching loss of nearly 3.14 times of thatduring a high frequency steady operation. In addition, the IGBT thermaltime constant is typically less than 100 ms. Typically, the firstsinusoidal peak stresses the IGBT the most as compared to other peaks.Therefore, if the frequency ramp starts from a non-zero frequency, say 4Hz (i.e., about 1% of full speed frequency), the IGBT stress would bereduced significantly. In one embodiment, a frequency ramp from anon-zero frequency reduces peak IGBT switch thermal stresses not only ifrotor position is known and fed back to the motor controller, but alsoif the rotor position is unknown and an open-loop generator startalgorithm is utilized. By implementing the frequency ramp methodsdescribed herein, IGBT switch thermal stresses are reduced significantlyso that the IGBT switch size increase would not be necessary for enginestart duty, hence reducing the aircraft weight.

Referring again to FIG. 9, the exemplary frequency ramp methods can beimplemented in the three-level inverter IGBT switch array 900. In oneembodiment, the frequency ramp method is implemented to spread the powersemiconductor (e.g. IGBT) thermal stress more evenly during anelectrical start of the aircraft engines.

In one embodiment, the IGBT peak junction temperature is relieved byramping up the motor controller output frequency from a non-zerofrequency. By starting from a non-zero frequency, (e.g., 4 Hz),localized heating is reduced at one particular IGBT by shifting the heatbetween different IGBT's in time. As such, the IGBT's are heated moreevenly. In one embodiment, where sensorless starting of an enginethrough a synchronous motor (generator used as motor) the frequency rampmethod can be implemented. At the beginning of the engine start, whenthe rotor position is unknown, starting from a non-zero frequencygenerates an induction torque through damper windings. This inductiontorque is always in forward direction and aids the initial polealignment and acceleration. Depending on the capability of the damperwinding, different initial frequencies could be used, a 1% startingfrequency (i.e., about) 4 Hz gives 1% slip, and an 8% starting frequencywould give 2% slip, for example. In the example, compared to starting at0 Hz, starting from 4 Hz would result in a 10° C. IGBT junctiontemperature reduction, and from 8 Hz would result in a 20° C.temperature reduction.

When necessary for the purpose of rotor pole alignment, a dwell time atthe initial non-zero frequency can also be introduced to give time forthe rotor to catch up with the stator field, and pull intosynchronization by synchronous torque. In addition, by using thisnon-zero initial frequency ramp, and the dwell, a motor controllerdesigned for other motoring load applications can be used for enginestart without increasing the IGBT size.

FIG. 11 illustrates a flowchart for a method 1100 of applying afrequency ramp to a motor controller in accordance with exemplaryembodiments. At block 1110, during engine startup implementing a motorcontroller, an appropriate frequency ramp as described herein isprogrammed into the motor controller.

The motor controllers described herein can be any suitablemicrocontroller or microprocessor for executing the instructions (e.g.,on/off commands) described herein. As such, the suitable microcontrolleror microprocessor can be any custom made or commercially availableprocessor, a central processing unit (CPU), an auxiliary processor amongseveral processors, a semiconductor based microprocessor (in the form ofa microchip or chip set), a macroprocessor, or generally any device forexecuting software instructions.

As will be appreciated by one skilled in the art, aspects of the presentinvention may be embodied as a system, method or computer programproduct. Accordingly, aspects of the present invention may take the formof an entirely hardware embodiment, an entirely software embodiment(including firmware, resident software, micro-code, etc.) or anembodiment combining software and hardware aspects that may allgenerally be referred to herein as a “circuit,” “module” or “system.”Furthermore, aspects of the present invention may take the form of acomputer program product embodied in one or more computer readablemedium(s) having computer readable program code embodied thereon.

Aspects of the present invention are described below with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems) and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer program instructions. These computer program instructions maybe provided to a processor of a general purpose computer, specialpurpose computer, or other programmable data processing apparatus toproduce a machine, such that the instructions, which execute via theprocessor of the computer or other programmable data processingapparatus, create means for implementing the functions/acts specified inthe flowchart and/or block diagram block or blocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat, in some alternative implementations, the functions noted in theblock may occur out of the order noted in the figures. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts, or combinations ofspecial purpose hardware and computer instructions.

In exemplary embodiments, where the methods are implemented in hardware,the methods described herein can implemented with any or a combinationof the following technologies, which are each well known in the art: adiscrete logic circuit(s) having logic gates for implementing logicfunctions upon data signals, an application specific integrated circuit(ASIC) having appropriate combinational logic gates, a programmable gatearray(s) (PGA), a field programmable gate array (FPGA), etc.

Technical effects include a reduction in switch ratings high power motorcontrollers resulting in significant weight & size savings, keyrequirements for aerospace applications.

While the invention has been described in detail in connection with onlya limited number of embodiments, it should be readily understood thatthe invention is not limited to such disclosed embodiments. Rather, theinvention can be modified to incorporate any number of variations,alterations, substitutions or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. Additionally, while various embodiments of the invention havebeen described, it is to be understood that aspects of the invention mayinclude only some of the described embodiments. Accordingly, theinvention is not to be seen as limited by the foregoing description, butis only limited by the scope of the appended claims.

The invention claimed is:
 1. A thermal stress reduction method for an aircraft motor controller having a three-level inverter switch array, and configured to control an electric power generator coupled to an aircraft engine, the method comprising: ramping the electric power generator to start an aircraft engine; during a time period associated with an aircraft engine start sequence: toggling the three-level inverter switch array to a three-level pulse width modulation (PWM) mode; determining if a first time interval in the three-level PWM mode exceeds a predetermined three-level PWM mode interval; toggling the three-level inverter switch array to a two-level PWM mode, in response to the first time interval exceeding the three-level PWM mode interval; determining if a second time interval in the two-level PWM mode exceeds a predetermined two-level PWM mode interval; and toggling the three-level inverter switch array to the three-level PWM mode, in response to the second time interval exceeding the two-level PWM mode interval, wherein the three-level inverter switch array includes a first switch, a second switch, a third switch and a fourth switch, wherein the first switch, the second switch, the third switch and the fourth switch are configured in a serial arrangement, the three-level PWM mode includes alternating between switching on the first switch and the third switch while applying a PWM signal to the second switch and the fourth switch, and switching on the second switch and the fourth switch while applying a PWM signal to the first switch and the third switch, the two-level PWM mode includes turning on the first switch and the fourth switch while applying a PWM signal to the second switch and the third switch, and a carrier frequency in the two-level PWM mode is greater than a carrier frequency in the three-level PWM mode.
 2. The method as claimed in claim 1 wherein during the three-level PWM mode, the first and fourth switches dissipate a greater amount of thermal energy relative to the second and third switches.
 3. The method as claimed in claim 2 wherein during the two-level PWM mode, the second and third switches dissipate a greater amount of thermal energy relative to the first and fourth switches.
 4. The method as claimed in claim 1 wherein a ratio of three-level PWM mode interval to the two-level PWM mode internal is inversely proportional to a ratio of a three-level PWM mode thermal loss to a two-level PWM mode thermal loss.
 5. An engine start system, comprising: an electric power generator having a three-level inverter switch array; an aircraft engine coupled to the electric power generator; an aircraft motor controller coupled to the electric power generator, the aircraft motor controller configured to: ramp the electric power generator to start the aircraft engine; during a time period associated with the aircraft engine start sequence: toggle the three-level inverter switch array to a three-level pulse width modulation (PWM) mode; determine if a first time interval in the three-level PWM mode exceeded a predetermined three-level PWM mode interval; toggle the three-level inverter switch array to a two-level PWM mode, in response to the first time interval exceeding the three-level PWM mode interval; determine if a second time interval in the two-level PWM mode exceeded a predetermined two-level PWM mode interval; and toggle the three-level inverter switch array to the three-level PWM mode, in response to the second time interval exceeding the two-level PWM mode interval, wherein the three-level inverter switch array includes a first switch, a second switch, a third switch and a fourth switch, wherein the first switch, the second switch, the third switch and the fourth switch are configured in a serial arrangement, the three-level PWM mode includes alternating between switching on the first switch and the third switch while applying a PWM signal to the second switch and the fourth switch, and switching on the second switch and the fourth switch while applying a PWM signal to the first switch and the third switch, the two-level PWM mode includes turning on the first switch and the fourth switch while applying a PWM signal to the second switch and the third switch, and a carrier frequency in the two-level PWM mode is greater than a carrier frequency in the three-level PWM mode.
 6. The system as claimed in claim 5 wherein during the three-level PWM mode, the first and fourth switches dissipate a greater amount of thermal energy relative to the second and third switches.
 7. The system as claimed in claim 6 wherein during the two-level PWM mode, the second and third switches dissipate a greater amount of thermal energy relative to the first and fourth switches.
 8. The system as claimed in claim 5 wherein a ratio of three-level PWM mode interval to the two-level PWM mode internal is inversely proportional to a ratio of a three-level PWM mode thermal loss to a two-level PWM mode thermal loss.
 9. The method of claim 1, wherein the predetermined three-level PWM mode interval is less than a semiconductor switch thermal time constant of the three-level inverter switch array.
 10. The engine start system of claim 5, wherein the predetermined three-level PWM mode interval is less than a semiconductor switch thermal time constant of the three-level inverter switch array. 